This invention relates generally to an inspection mechanism for detecting flaws in tubing and more specifically relates to a probe carrier drive assembly for moving a probe carrier without slip or creep in a steam generator tube so that a probe, which has an inspection scanning device attached thereto and which is connected to the probe carrier, selectively rotates in place in the tube, follows a linear scanning path through the tube or follows a helical scanning path having a variable pitch.
It is well known that a steam generator is a device for generating steam when heat is transferred by conduction through a heat conductor boundary separating a primary fluid from a secondary fluid, wherein the secondary fluid is water and wherein the primary fluid obtains a higher temperature than the secondary fluid. As the temperature of the secondary fluid increases, the secondary fluid reaches saturation temperature beyond which saturation temperature increasing fractions of the secondary fluid enter the vapor phase thereby producing steam. Typically, the steam generator includes a plurality of conduits or tubes through which the primary fluid flows, the walls of which tubes function as the heat conductor boundary for conducting heat from the primary fluid to the secondary fluid.
In a nuclear reactor the primary fluid flowing in the steam generator tubes is radioactive water; hence, the steam generator is designed such that the radioactive primary fluid does not radioactively contaminate the secondary fluid by intermixing with the secondary fluid. It is therefore desirable that the tubes remain leak-tight so that radioactive primary fluid remains everywhere separated from the secondary fluid to avoid intermixing the radioactive primary fluid with the secondary fluid.
Occasionally due to tube wall defects or tube wall cracking caused by stress and corrosion during operation, the steam generator tubes may develop surface and volume flaws and thus may not remain leak-tight. More specifically, laboratory tests have indicated that the defects or cracking referred to above may be due to a combination of the high temperature of the primary fluid, the conditions of stress and strain resulting from hard rolling the tubes and a possible susceptibility of the tubing material microstructure to experience intergranular stress and corrosion during operation. If through wall cracking occurs, some of the steam generator tubes may not remain leak-tight. Therefore, it is customary to inspect the tubes for flaws or irregularities so that corrective action may be taken to ensure that the primary fluid does not intermix with the secondary fluid. Such corrective action may be to plug or sleeve the tubes having flaws or irregularities.
However, before corrective action is taken it is prudent to first determine which steam generator tubes have flaws or irregularities. As well known in the art of nondestructive examination, determination of which tubes have flaws or irregularities requiring corrective action may be performed by inspecting the suspect tubes using an eddy current and/or ultrasonic transducer inspection device which is capable of electronically and/or sonically scanning the suspect tube. When an ultrasonic transducer is used, the ultrasonic transducer is coupled to the tube wall by a suitable couplant, such as water. The ultrasonic transducer signals, which pass through the couplant, are then reflected from the inner and outer surfaces of the tube wall and returned to the transducer and converted to electrical impulses which are transmitted to a measuring device. The reflected signals from the inner and outer surfaces of the tube wall are spaced apart in time by a time interval proportional to the thickness of the tube wall. The measuring device converts this difference in time to a voltage level indicating the thickness of the tube wall. The voltage level is then output to a display device for displaying the variation in tube wall thickness at various locations along the tube wall. Eddy current techniques, on the other hand, are based on the well known principle that when an electrical conductor is placed in an alternating magnetic field, eddy currents are generated in the conductor by electromagnetic induction. The magnitude and phase of these currents are a function of the electrical conductivity and physical characteristics of the conductor. These eddy currents produce a magnetic field which may be detected and measured. Thus, an eddy current probe carrier, which includes a test coil to which an oscillating current is applied, is moved along the tube and the effect on the electrical impedance of the test coil is measured to provide an indication of the physical characteristics of the tube. Of course, to scan the tube in a predetermined manner for flaws or irregularities the ultrasonic transducer and/or eddy current device should be suitably moved in the predetermined scanning pattern along the inside surface of the tube longitudinally within the tube.
It is the usual practice in the art to include the ultrasonic transducer and/or eddy current device in a probe connected to an elongated probe carrier, which probe and probe carrier are capable of being inserted into and moved along the inside surface of the tube to be inspected. The probe carrier is in turn engaged by a probe carrier drive which may engage the probe carrier by friction rollers. However, the use of friction rollers can subject the probe carrier to slip and creep; thus, a problem in the art has been to provide a probe carrier drive that allows the probe to be accurately moved in the desired manner without slip or creep within the tube to be inspected.
Moreover, another problem in the art has been to provide a probe carrier drive capable of engaging the probe carrier such that the probe carrier and connected probe selectively rotate in place in the tube, follow a linear scanning path in the tube or follow a helical scanning path having a variable pitch. Moving the probe carrier and probe in this manner allows the desired amount of data regarding the thickness of the tube wall to be obtained. Although the prior art may disclose probe carrier drives which allow the probe carrier and probe to rotate in place, to follow a helical scanning path or to follow a linear scanning path, a problem frequently encountered in the art is to provide a probe carrier drive which allows the probe carrier and probe to follow a helical scanning path having a variable pitch and to provide a probe carrier which moves within the tube without slip or creep.
There are several devices known in the art for moving a probe carrier and probe in a tube. One such device is disclosed by U.S. Pat. No. 3,831,084 issued August 20, 1974 in the name of Joseph J. Scalese et al. and entitled "Probe Carrier With Means For Selectively Permitting A Stationary Or A Helical Scan". This patent discloses a helically scanning eddy current flaw detector having a controllable sleeve which allows the detector to selectively either follow a helical scanning path or rotate in place. However, the Scalese et al. device does not appear to allow the detector to follow either a linear scanning path without helical motion or to follow a helical scanning path having a variable pitch.
Another device for moving a probe carrier in a tube is disclosed by U.S. Pat. No. 3,926,040 issued Dec. 16, 1975 in the name of Thomas E. Cowell and entitled "Device For Guiding Sensor Movement Within A Tube". The Cowell device relates to precisely repositioning a sensor within a tube such as a nuclear reactor vessel component in order to accomplish nondestructive testing, such as inspection of the tubular interior wall for flaws. The Cowell device comprises an elongated carrier pipe extending through a drive gear and further comprises a working head secured to the carrier pipe. A first reversible motor is operatively coupled with the drive gear to cause rotation of the carrier pipe. A second reversible motor is connected to the drive gear and operatively coupled to a gear rack to selectively cause axial movement of the carrier pipe. Actuator means is provided to operate the first reversible motor and second reversible motor either separately or simultaneously to thereby cause movement of the carrier pipe and repositioning of the working head within the tube. The Cowell device, however, does not appear to allow the carrier pipe to follow a helical scanning path having a variable pitch in the manner of the present invention.
Yet another device for moving a probe carrier in a tube is disclosed by U.S. Pat. No. 4,624,400 issued Nov. 25, 1986 in the name of John J. Zimmer entitled "Electromagnetic Probe Carrier Drive Apparatus" and assigned to the Westinghouse Electric Corporation. The Zimmer patent is directed towards an apparatus for driving a probe connected to an elongated flexible probe carrier. The apparatus comprises a rotatable drive member having an endless drive surface and means for holding the probe carrier in frictional engagement with the drive surface. However, the Zimmer patent does not appear to disclose means for rotating the probe carrier in place or moving the probe carrier without slip or creep in a helical scanning path having a variable pitch.
Consequently, while the prior art discloses devices for moving a probe carrier and connected probe in a tube, the prior art does not appear to disclose a device which moves a probe carrier without slip or creep in a tube such that the probe carrier and probe selectively rotate in place, follow a linear scanning path, or follow a helical scanning path having a variable pitch.
Therefore, what is needed is a probe carrier drive assembly for moving a probe carrier and probe without slip or creep in a steam generator tube so that the probe, which has an inspection device attached thereto and which is connected to the probe carrier, selectively rotates in place, follows a linear scanning path through the tube, or follows a helical scanning path having a variable pitch.